Integrated circuit devices are commonly used to generate output signals which are supplied from an output terminal or pad to a load circuit on a connected printed circuit board. The high speed transmission of signals from an output pad of an IC chip to a load on a printed circuit board provides problems. Each time a signal is driven off the chip, the external load capacitance must be discharged or charged via the output driver circuitry and the inductance of the voltage supply lines. The inductance may be due to conventional features such as bond wires and lead frame. The capacitance which has to be driven by the driver circuitry may be formed from a mixture of printed circuit board track capacitance, load capacitance of the device on the printed circuit board and other integrated circuit input or output capacitance. When an integrated circuit device is connected to a load on a printed circuit board the ground and power supply lines on the integrated circuit chip as well as the output pad connected to the load circuit on the printed circuit board will each be connected to circuits including inductance and capacitance such that resonance occurs when transmitting output signals from the output pad. Oscillations in voltage caused in this way may cause temporary rise and fall in the voltage on the on chip ground and supply lines herein referred to as supply bounce. Such supply bounce has undesirable effects such as the injection of noise into sensitive analog circuits. The speed of switching of transistors in the output driver circuit will vary from chip to chip because of variations in the integrated circuit processing during manufacture and also due to variations in operating temperature. The supply bounce will be greatest if the particular chip has fast transistors whereas the signal propagation delay will be greatest if the chip has slow transistors. The design of output driver circuits is therefore a compromise between the speed of the output driver circuits and the acceptable level of on chip supply bounce.
The supply bounce is dependent on the rate of change of drain current in the transistors of the output driver circuits.
It is an object of the present invention to provide circuits and methods of controlling the rate of change of drain current in a switching transistor.
It is also an object of the present invention to provide a means of assessing the transconductance (that is rate of change of drain current with change in gate voltage) of a transistor.